4
 = 
8
x
20

Cross multiply:
4 * 20 = x * 8

Simplifying
4 * 20 = x * 8

Multiply 4 * 20
80 = x * 8

Reorder the terms for easier multiplication:
80 = 8x

Solving
80 = 8x

Solving for variable 'x'.

Move all terms containing x to the left, all other terms to the right.

Add '-8x' to each side of the equation.
80 + -8x = 8x + -8x

Combine like terms: 8x + -8x = 0
80 + -8x = 0

Add '-80' to each side of the equation.
80 + -80 + -8x = 0 + -80

Combine like terms: 80 + -80 = 0
0 + -8x = 0 + -80
-8x = 0 + -80

Combine like terms: 0 + -80 = -80
-8x = -80

Divide each side by '-8'.
x = 10

Simplifying
x = 10